Hi guys!
Have bought the above mentioned USB wifi dongle but having issues.
My system is all ~amd64, all packages up to date. Using networkmanager for the connections.
My kernel config is exactly as suggested here: https://forums.gentoo.org/viewtopic-t-953924-start-0.html
Also the firmware is being loaded.
Networkmanager is showing as connected for a short moment, but then switches to offline soon after.
Here is what the syslog says. Any suggestions appreciated!

Hello!
The failure was without dhcpcd. Enabling it doesn't make much difference, only that the networkmanager applet in KDE switches on/off several times.
There are no more interfaces under /etc/init.d. Only "net" which seems to be auto-generated by Networkmanager._________________"Coincidence is God's way of remaining anonymous."
Albert Einstein
"The road to success is always under construction"

do you use networkmanagement or nm-applet to configure the network?
you have emerged wpa_supplicant and it is not in a run level? wpa_supplicant should be using the nl80211 driver though this nic will work with wext if wext kernel support is enabled.
you should have /etc/init.d/net.lo

Thanks for your input. I tried all the solutions from the above given thread. None fixed it for me.
First, I removed everything from /etc/init.d and cleared the runlevels. Only Networkmanager running.
After that gave no success, I manually started dhcpcd and restarted Networmanager again. Still nothing.
Below is what dmesg shows:

So, what did you do? I am on latest ~amd64 version (on Windows right now, so I can't check the version).
Did any version proof to work for you?_________________"Coincidence is God's way of remaining anonymous."
Albert Einstein
"The road to success is always under construction"

Nah, sh**. Doesn'T work for me either
I emerged dhcpcd-5.9.9.7-r1 but still the same.
Even if I stop Networkmanager completely and start up wpa_supplicant, I can't configure it in the gui. The network and adapter fields are just empty, scanning doesn't show up anything either.
ifconfig shows wlan0 there.
I tried it with sysreccd and there it connects flawless... Whew!_________________"Coincidence is God's way of remaining anonymous."
Albert Einstein
"The road to success is always under construction"

Just installed network manager for my ID 148f:5370 Ralink Technology, Corp. RT5370 Wireless Adapter and it is working beautifully with gentoo-sources 3.8.13 and 3.10.0.
if you have wired internet connectivity when gentoo is booted, boot gentoo. If no connectivity, boot install cd/usb, mount the gentoo partitions, enter the chroot.

Given that I have made no other changes to my system (other than updating) and that I have a relatively simple configuration I am going to put this down to a bug. I am unsure in which package, however. But, manually su'ing to root and running dhcpcd eth1 gets me connected.

have a suspicion based on experiences recently that things have been "fixed" so that only one up interface can exist at a time. ifplugd or netplug sometimes resolve the problem. stopping one interface and restarting the other sometimes resolves the problem._________________Defund the FCC.

Guys, thanks for your effort. What DONAHUE suggests:
I don't really have the time right now troubleshooting my new box. I just copied over my old core i7 installation from my laptop over to the haswell-beast, did some kernel tweaking and an emerge -e @world, and see, it works. This network issue is giving me grey hairs. So far my Linux-experience with this brand new device has been ugly:
Sabayon: no mouse and keyboard. Kubuntu doesn't boot. MY favorite, Gentoo: this issue. Sysrescd works pretty good, but is not what I want to install by any means.

dhcpcd wlan0 only gives this erros (with or without networkmanager running):

have a suspicion based on experiences recently that things have been "fixed" so that only one up interface can exist at a time. ifplugd or netplug sometimes resolve the problem. stopping one interface and restarting the other sometimes resolves the problem.

Topic is solved. It had nothing to do with a misconfiguration but was a Networkmanager bug. Latest version fixed it. Thanks to all participants!_________________"Coincidence is God's way of remaining anonymous."
Albert Einstein
"The road to success is always under construction"